220
C. E. MESSER,E. B. DAMON, P. C. MAYBURY, J. MELLORAND R. A. SEALES
Vol. 62
The values of C a at 1313°K. for the various catalysts as calculated by equation 12 are given in Table V.
have been attained at the lower temperatures if the catalysts had not been heated to the higher temperatures. No attempt was made to use mixed oxides nor to try to produce a catalyst of particTABLE V CALCULATED CONCENTRATION O F ADSORBED MOLECULES ularly high efficiency. The results of this investigation show, however, that the decomposition. of OF NO nitric oxide on solid surfaces is a suitable reactlqn Ck Catalyst molecules/cm. 1 for studying heterogeneous reactions and catalyt~c A1203 1.3 X 10'O activity and that there may be a chance of finding CaO 0.96 x 109 a suitable and permanent catalyst for decomposing 0.77 X 108 Gaz03 nitric oxide a t considerably lower temperatures ZrOa 2.5 X 108 than those used here. It is hoped that chemists, ZnO 1.1 x 108 who are interested in heterogeneous catalysis, will The concentration of adsorbed NO molecules on study this reaction further and perhaps find a mawhich can decompose the small amounts of A1203 is about 100 times the concentration of mole- terial oxides (< 0.1%) in the exhaust gases of autocules adsorbed on ZnO at 1313°K. The rate of de- nitric mobiles and power plants. At these low concentracomposition of NO on A1208 as given in Table IV is tions of nitric oxide the reaction would probably also of this order of magnitude larger than the rate not be zerporder. In view of the apparently low on ZnO while the activation energies are the same. Thus the higher concentration of adsorbed mole- activation energy on iron oxide a specially prepared cules on A12O3 may account for the faster rate of iron oxide might show promise at lower temperatures. decomposition. Acknowledgment.-The authors are pleased t o In view of the over-all data, CaO, A1203 and GazOs seem to be the best catalysts for the decom- acknowledge financial support from the Wisconsln position of nitric oxide. A1203 has a large surface Alumni Research Foundation granted by the area which offsets the higher activation energy and Research Committee of the Graduate School of Gaz03has a low activation energy which offsets the the University of Wisconsin. They wish to thank smaller surface area. With calcium oxide both the E. I. du Pont de Nemours Company for the surface area and the activation energj are large financial aid granted during the summer of 1956. They wish to acknowledge also the preliminary, and the catalytic effect is large. It should be emphasized that in some cases the qualitative work on the catalytic decomposition temperatures are high enough to cause sintering of of nitric oxide carried out in this Laboratory by the surfaces and that greater catalytic effects could Dr. R. J. Williams and Dr. E. L. Yuan. t
SOLID-LIQUID EQUILIBRIUM IN THE LITHIUM-LITHIUM HYDRIDE SYSTEM BY CHARLES E. MESSER,EDWINB. DAMON, P. CALVINMAYBURY, JOHN MELLOR AND REGINA A. SEALES Contribution Number 248 from the Department of Chemistry, Tufts University, Medford 65, Mass. Received September 16, 1067
The.freezing points of mixtures of lithium hydride with lithium metal have been determined by the method of thermal analysis from 13 t o 99.8 mole % lithium hydride. Results obtained by removing hydrogen from lithium hydride agreed with those obtained by adding hydrogen t o lithium metal. The apparatus is described. The freezing point of 99.8 mole.% LiH was 688 i 1". The freezing point as a function of composition resembled in behavior that of metal-metal halide systems, dropping t o a monotectic of 685 f 1' from 26 to about 98 mole % LiH and then dropping sharply with further hydrogen removal. *
Introduction Lithium hydride is the only metal hydride known to melt reversibly under a hydrogen pressure of less than one atmosphere. Its melting point was first reported by Guntzl at G80°,and this value has been confirmed by later workers. The dissociation pressure of hydrogen over mixtures of lithium and lithium hydride was investigated by Hurd and Moore12Hillla Perlow4and most recently by Heumann and S a l m ~ n . ~ The isotherms
follow the typical simple metal-hydrogen system behavior.6 The solid-liquid phase equilibrium diagrams of the sodium-sodium halide systems were studied by Bredig, Johnson and Smith.' The lithium-lithium hydride system should behave like the sodium-sodium halide system, since both the hydrides and the halides are ionic in character and both have the same face-centered cubic structure. Experimental Apparatus.-The
method of thermal analysis was used.
(1) A. Guntz, Compt. rend., 193, 694 (1896).
B. Hurd and G. A. Moore, J. A m . Chem. SOC..67, 332 (1935). (3) L. L. Hill, Thesis, University of Chicago, 1938. (4) M. R. J. Perlow. Thesis, University of Chiorsgo, 1941. (5) F. K. Heumann and 0. N. Salmon, Report USAEC-KAPL-1667, December I, 1956. (2) C.
(6) G. G. Libowitz and T.R. P. Gibb, Jr., THIEJOURNAL, 81, 793 (1957). (7) M. A. Bredig, J. W. Johnson and W. T. Smith, J . Am. Chem. Xoo., 77, 307 (1955). (8) G . N.Lewie, ibid., 38, 762 (1916).
'
SOLID-LIQUID EQUILIBRIUM IN THE LI-LIH SYSTEM
Feb., 1958
The core of the apparatus is shown in Fig. 1. The sample is in a bomb B of stainless steel 316 which is held vertically in a standard tube furnace of 12" length. The bottom of the bomb contains a thermocouple well C with two chromelalumel thermocouple junctions: No. 1 connected directly to a Brown Elektronik recording potentiometer, and No. 2 connected through an ice junction t o a thermocouple potentiometer. The bomb is sealed by the copper ring gasket G which is' forced against the rim of the bomb by the cap H, set screws I, and discs J. The valve K enables the bomb contents to be transferred from dry-box to apparatus anaerobically. The fitting L enables the bomb to be connected through a glass-metal seal to a standard high vacuum system with manometer, calibrated volumes, and hydrogen source. The furnace temperature could be held constant by a photoelectric cell-galvanometer arrangement actuated by the difference in e.m.f. between thermocouple 2 in the bomb and an equal and opposite imposed e.m.f. The same control system was used for heating and cooling rates by means of the difference couple 3, 4 (Fig. 1). Materials.-The lithium metal was obtained from the Maywood Chemical Works, Maywood, N. J. It was their "low sodium'' grade of a type reported by Heumann and Salmon' to assay 99.75% Li and to contain only 0.02% Na. It was trimmed of its oxide-nitride coating in an argon-filled dry-box before use. Linde hydrogen was purified by synthesis and subsequent decomposition of uranium hydride. Lithium hydride was made by treating lithium metal with purified hydrogen for 15 hours a t 725" and 1 atmosphere. All transfer operations were carried out under argon. The hydride was analyzed by hydrogen evolution on hydrolysis with water, and then titration of the resulting base. The results are shown in Table I.
221 ONE INCH
K
.c--c
TABLE I ANALYSIS OF LITHIUM HYDRIDE SAMPLES Sample no.
H, %
Li,%
% of theoret. H
1 2 3 Stoicli
12. 65 12.65 12.60 12.68
87.14 87.12 87.03 87.32
99.8 90.8 90 B
Method.-In the series of runs designated as 1, 2 and 3, weighed amounts of lithium hydride of samples no. 1, 2 and 3, respectively, were introduced into the sample bomb in an argon-filled dry-box. The bomb was capped, closed, removed from the dry-box and connected to the system. Following determination of the freezing point of the original hydride by a series of cooling curves under 1 atmosphere of hydrogen, measured amounts of hydrogen were removed at 725", and one or more freezing curves were run after each removal. The cooling rate was adjusted t o about 1' per minute just before the beginning of freezing. The process was continued until the freezing point became indistinguishable on the recorder traces. The sample was finally rehydrided until the freezing point was raised to that of the original hydride. Sample size was about 8 g. of lithium hydride, or 7 g. of lithium. I n the series of runs designated as 4 and 5, weighed amounts of lithium were allowed to absorb measured quantities of hydrogen a t 725". The final addition of hydrogen was carried out a t 1 atmosphere until the freezing point went up no further, and the total amount absorbed was stoichiometric within experimental error. Only traces of volatilized deposit were noted in these runs. Composition Measurements.-In general the compositions could be calculated from the gas measurements with a precision of k 0 . 2 5 mole % lithium hydride. The change of composition on dehydriding samples 1, 2 and 3 could be measured to i0.02'% over the range of 98-99.8 mole % lithium hydride. The hydrogen diffusion loss through the steel bomb was measured as 0.030 i 0.005 cm.3 S.T.P./min. atm.1'2 a t 720°, and the small correction was applied. I n runs 1, 2 and 3, there was a considerable deposit of volatilized material in the upper part of the bomb at the end, shown by analysis to be mostly lithium metal. It was not possible to correct the composition for this volatilization. A t compositions approaching pure lithium hydride,
3 i 2 4
Fig. 1.-Apparatus: A, furnace; B, sample bomb; C, thermocouple well; D, Vitreosil tube sealed with Sauereisen cement; E, radiation baffles; F, cooling coil; G, co per ring gasket; H, cap; I, set screws; J, discs; K, valve; ! I, fitting. where the high pressure of hydrogen lowers the pressure of lithium vapor, it is felt that the precision of i0.250/, is still valid. Below 90 mole %, down to 45%, the compositions of samples 2 and 3 may be in error by several per cent., but are qualitatively of value in establishing the constancy of the plateau tem erature (Fig. 2). Below 45%, where runs 2 and 3 difkred widely, the results were totally rejected. Temperature Measurements .-The precision of temperature measurements may be determined by the precision in each of the following: in the calibration the standard couple against the aluminum point, f 0 . 1 5 ; in the calibration of the check couple (No. 2) in place against the standard, f0.25"; in the precision of the calibration of the check couple against the recorder, &0.25", and in the constancy of temperature over the region of the sample in the bottom three inches of the bomb, i 0 . 2 5 ' . This gives an over-all precision of &0.5", which is borne out by the results in Table 11. Due to indeterminate errors the accuracy is probably not better than f 1 " .
05
Results The entire freezing point-composition curve is shown in Fig. 2, and the region of high hydrogen content in Fig. 3. The measurements made by
C. E. MESSER,E. B. DAMON, P. C. MAYBURY, J. MELLORAND R. A. SEALES
222 690 680
d u-
670
I
FREEZING POINT A
640
Type of run
F.p; LiH,
C.
Monotectic, OC.
Depression,"C.
... ... 2.9 688.0 684.9 3.1 3 687.9 686.2 1.7 688.1 684.1 4.0 4 5 687.0 684.5 2.5 Mean .... 687.8 684.9 2.9 a Dehyd. = dehydriding of LiH; Hyd. = hydriding of Li metal. Absolute temperatures cannot be given for sample no. 1 because of an uncertainty in the thermocouple calibration. 1
'
2
.-8 6 5 0 h
TABLE I1 MONOTECTIC, LITHIUM-LITHIUM
~ D
HYDRIDE SYSTEM Sample no.
.ga 660 3
Vol. 62
-
10
30 40 50 60 70 80 90 Mole % LiH. Fig. 2.-Freezing point us. composition, lithium-lithium hydride system: 0 0 samples 2, 3, hydrogen removed from lithium hydride; @ 8 samples 4, 5, hydrogen added to lithium r-ietal. 20
/i
92.0
94.0 96.0 98.0 100.0 Mole yo LiH. Fig. 3.-Freesing point lowering us. composition, lithiumlithium hydride system, region of high hydrogen content: 0,sample 1; 0 , sample 2.
removing hydrogen agree closely with those made on adding hydrogen. The freezing point of near-stoichiometric lithium hydride is found to be 688 f 1". The removal of hydrogen lowers the freezing point to a monotectic a t 685", the temperature remaining constant a t this value for all mixtures between about 26 and 98 mole yo lithium hydride. Further removal of hydrogen lowers the freezing point again, down to G24" a t 13 mole % lithium hydride, the lowest hydrogen content a t which there was a detectable break in the freezing curves. The freezing points and moiiotectics are shown in Table 11. The curve below the lower plateau limit of 26 mole yoLiH may be considered as the curve of solubility of lithium hydride in lithium as a function of temperature. It may be represented with an average deviation of *0.8yo in coniposition by the equation log
l~/(l
+
9 ~ )
-3881/T
This corresponds to AH =
+ 2.835
+ 15.74 kcal./mole for
Dehyd." Dehyd. Dehyd. Hyd." Hyd.
b
b
the heat of solution of lithium hydride in liquid lithium saturated with hydrogen a t the plateau dissociation pressure of the lithium-hydrogen system. The equilibrium dissociation pressure was checked at 700" at 37.0 and 64.8 mole yo lithium hydride on sample no. 5. One hour was sufficient to establish equilibrium. The values found were 28.8 and 27.8 mm., respectively, agreeing well with Heuinann and Salmon's plateau pressure of 28 =k 1 mm. Discussion The general behavior of the system agrees with that of the sodium-sodium halide systems.' The nionotectic depression of 3" is smaller than the 5-9" values for these systems. The solubility of lithium hydride in lithium a t the monotectic, 26 mole %, is greater than the largest solubility in these systems, that of sodium fluoride in sodium of 17%. No results on the solubility of lithium halides in lithium seem to be in the literature. Figure 3 shows that in the 96 to 98.5 mole % region the discrepancies in the freezing points are greater than those across the rest of the plateau. I n fact, within experimental error, it is not possible to decide whether the plateau ends sharply at 9898.5%, or whether a gradual rise in freezing point sets in near 96%, followed by a sharper rise between 98.5 and 99%. This behavior may be explained by some sluggishness in the achievement of phase equilibrium which does not exist well along the monotectic. Actually for certain conipositions in this region there should be two breaks in the freezing curves: one above the monotectic corresponding to the appearance of the solid lithium hydride phase, and a second a t the monotectic corresponding to the appearance of the liquid lithium phase. Within experimental error only one break could be observed. Probably the 1" per minute cooling rate was too fast to enable the two processes t o occur separately within this narrow 3" interval. Acknowledgment.-This research was carried out under the sponsorship of the Atomic Energy Commission, Contract AT(30-1) 1410.
